Double Taxation Germany/Netherlands

By default, your entire world income is taxable here, while you are residing in Germany (regardless of additional taxation elsewhere).
This default can be overwritten by bilateral tax treaties, which Germany has with all EU countries (among others).
In most such treaties, some income is defined as taxable (only) in the source country. In most cases, pay for work or services is taxable where the work is performed (and not where the customer is located).
This is regardless of where and how you are paid.
So you should check the Germany-NL tax treaty. A professional tax consultant is also advisable, as doing things wrongly (also unknowingly) can be a criminal offense!
(Remark: In any case, you have to list your entire world income in your yearly German tax declaration, because it's their decision, not yours, what is taxable and what not. Furthermore, even non-taxable income can have an effect on your tax rates on taxable income.)